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American River Watershed Common Features 2016 – Sacramento Weir and Bypass

The development of a system-wide flood management plan in the Sacramento Valley began around 1850. Up until the flood of 1909, flood management activities focused primarily on confining flows within the existing river channels. This was a period with frequent levee failures, including failures in the 1909 flood event. As a result, the State of California and federal government decided that a bypass system was needed to divert waters from the main rivers during high flows to relieve stress along the levee system. The state approved the Sacramento and Yolo Bypass systems as part of the Sacramento River Flood Control Project in 1911. Congress then authorized the U.S. Army Corps of Engineers (USACE) to construct the remainder of the project in 1917 and incorporated it into the federal system.

The Sacramento Weir, completed in 1916, is situated along the right bank of the Sacramento River between the Yolo Causeway and the Garden Highway, approximately 3 miles upstream from the confluence of the American River and Sacramento Rivers. Its primary purpose is to protect the City of Sacramento from excessive flood stages in the Sacramento River by diverting river flows west into the two-mile-long Sacramento Bypass that connects to the Yolo Bypass.  The Yolo Bypass takes 80% of the flood flows during storm events from major valley rivers including the Sacramento, American and Feather Rivers.

Just downstream of the Sacramento Weir is the confluence of the American and Sacramento Rivers. The design flood capacity of the American River is higher than that of the Sacramento River. During a major flood event, flows from the American River channel often exceed the capacity of the Sacramento River downstream of the confluence. When this occurs, floodwaters flow upstream from the mouth of the American River to the Sacramento Weir.

USACE and the State of California are planning to widen the Sacramento Weir and Bypass to allow more water to enter into the Bypass system during flood events, thereby reducing the water surface elevation in the Sacramento River. This work includes widening the existing weir by 1,500 feet and constructing a new 2-mile-long setback levee along the Sacramento Bypass. The first phases of construction began in 2020.

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